ASBOG - Association of State Boards of Geology Hydrogeology Questions and Answers — Questions and Answers
Question 1: A hydrogeologist needs to calculate the specific discharge (Darcy flux) through a uniform, saturated sand aquifer. The hydraulic conductivity (K) is 50 m/day, the hydraulic gradient (i) is 0.002, and the effective porosity (ne) is 0.25. The cross-sectional area (A) perpendicular to flow is 100 m². What is the specific discharge (q)?
- 10 m³/day
- 0.4 m/day
- 100 m/day
- 0.1 m/day (Correct answer)
Correct answer: 0.1 m/day
Specific discharge (q), also known as Darcy flux, is calculated using Darcy's Law: q = K * i. It represents the flow rate per unit area and has units of velocity (L/T). The formula does not include porosity or the total cross-sectional area. Therefore, q = 50 m/day * 0.002 = 0.1 m/day.
Question 2: Which of the following best distinguishes Transmissivity (T) from Hydraulic Conductivity (K) in an aquifer?
- Transmissivity is a property of the fluid, while Hydraulic Conductivity is a property of the porous medium.
- Transmissivity is a dimensionless coefficient, while Hydraulic Conductivity has units of length per time.
- Transmissivity represents the ability of the entire saturated thickness of an aquifer to transmit water, while Hydraulic Conductivity is an intrinsic property of the porous material per unit area. (Correct answer)
- Transmissivity is only applicable to unconfined aquifers, whereas Hydraulic Conductivity applies to both confined and unconfined aquifers.
Correct answer: Transmissivity represents the ability of the entire saturated thickness of an aquifer to transmit water, while Hydraulic Conductivity is an intrinsic property of the porous material per unit area.
Hydraulic Conductivity (K) is a measure of a material's capacity to transmit water through a unit cross-sectional area. Transmissivity (T) is the product of the hydraulic conductivity and the saturated thickness of the aquifer (T = K*b). Therefore, transmissivity quantifies the ability of the full thickness of the aquifer to transmit water.
Question 3: A contaminant plume is migrating through a sandy aquifer. Geochemical testing reveals that the contaminant strongly sorbs to organic carbon on the sand grains. This sorption process will primarily result in which of the following transport phenomena?
- Increased advection
- Enhanced dispersion
- Accelerated diffusion
- Retardation of the contaminant front (Correct answer)
Correct answer: Retardation of the contaminant front
Sorption is the process where dissolved contaminants adhere to solid particles in the aquifer matrix. This process slows down the movement of the contaminant relative to the average groundwater velocity. This slowing effect is known as retardation. The retardation factor (R) quantifies this effect, with higher values indicating slower contaminant movement.
Question 4: A pumping test is conducted in a homogeneous, isotropic, confined aquifer of infinite areal extent. When drawdown data from an observation well is plotted against the logarithm of time on a semi-log graph, what is the expected shape of the data curve after the initial effects of wellbore storage have dissipated?
- A straight line with a positive slope (Correct answer)
- A horizontal line indicating equilibrium
- A bell-shaped curve peaking at mid-time
- An exponentially decreasing curve
Correct answer: A straight line with a positive slope
According to the Cooper-Jacob method, which is a simplification of the Theis solution for later-time data, the drawdown in a confined aquifer is directly proportional to the logarithm of time. Therefore, when plotted on a semi-log graph (drawdown on the arithmetic axis vs. time on the logarithmic axis), the data should form a straight line. The slope of this line is used to calculate the aquifer's transmissivity.
Question 5: A well is drilled into a saturated geologic formation that is overlain by a low-permeability clay layer (an aquitard). The static water level in the well rises to an elevation significantly above the top of the saturated formation. This condition is characteristic of which type of aquifer?
- A perched aquifer
- An unconfined aquifer
- A confined (artesian) aquifer (Correct answer)
- A leaky unconfined aquifer
Correct answer: A confined (artesian) aquifer
A confined aquifer is a saturated formation bounded above and below by low-permeability layers (aquitards or aquicludes). The water within this aquifer is under pressure. When a well penetrates a confined aquifer, the water level will rise to the potentiometric surface, which is often above the top of the aquifer itself. This is also known as an artesian condition.
Question 6: The Theis non-equilibrium equation is a fundamental tool for analyzing pumping test data. Which of the following is a key assumption required for the valid application of this method?
- The aquifer is unconfined with delayed gravity drainage.
- The pumping rate varies logarithmically with time.
- The aquifer is homogeneous, isotropic, and of infinite areal extent. (Correct answer)
- Recharge from a nearby river is induced by pumping.
Correct answer: The aquifer is homogeneous, isotropic, and of infinite areal extent.
The Theis solution is an idealized mathematical model based on several critical assumptions. These include that the aquifer is homogeneous (uniform properties everywhere), isotropic (properties are the same in all directions), confined, of uniform thickness, and infinite in areal extent. It also assumes the well fully penetrates the aquifer and is pumped at a constant rate.
ASBOG - Association of State Boards of Geology Hydrogeology Questions and Answers
A hydrogeologist needs to calculate the specific discharge (Darcy flux) through a uniform, saturated sand aquifer.
The hydraulic conductivity (K) is 50 m/day, the hydraulic gradient (i) is 0.002, and the effective porosity (ne) is 0.25.
The cross-sectional area (A) perpendicular to flow is 100 m².
What is the specific discharge (q)?